This invention relates generally to a tyre removal machine, and more particularly to a pneumatically operated tyre removal machine.
With particular but not exclusive reference to wheels (wheel rim+tyre) for light motor vehicles, such as automobiles and vans, and motorcycles, tyre removal machines are known comprising basically a frame on which there are mounted a self-centering unit of vertical axis for locking the wheel rims, and a rearward column which supports, in a manner enabling it to be adjusted in both a vertical and horizontal direction, a tool for mounting and removing tyres onto and from the respective wheel rims.
In such known tyre removal machines the self-centering unit is rotated by a step-down gear coupled to the vertical shaft supporting said self-centering unit, and driven by an electric motor by way of a belt transmission.
In addition, these tyre removal machines are usually provided with a lateral arm which at one end is hinged to one side of the frame on a rear vertical axis, and at its other end supports a bead release tool.
Between said frame and said arm there is interposed a single-acting cylinder-piston unit which is coupled to the arm with unilateral engagement, the bead release tool being able to rotate by virtue of said arm, between a rest position in which it is spaced from the corresponding side of the frame, and a working position in which it makes contact with the bead of a wheel (wheel rim+tyre) resting in an upright position against said side.
In these described tyre removal machines, during the mounting and removal of a tyre the self-centering unit is rotated at constant speed by said geared motor unit, both if the tyre beads are easy to mount on and withdraw from the respective bead stop edges of the wheel rim and if this mounting and withdrawal require considerable force.
However the combination of this considerable force and the rotational speed of the self-centering unit, which as stated does not change when the resistance offered by the bead varies, means that the bead is subjected to a sharp sudden action which can result in, and indeed has resulted in, inconvenient damage to the bead, such as stretching and cracking.
In addition, with the aforedescribed tyre removal machines the tyres can also suffer damage during bead release.
This is particularly so if the bead strongly adheres to the bead stop edge and to its containing seat, and/or if this latter is provided on the side opposite the bead stop edge with a circumferential rib to prevent bead release if the tyre should become deflated while the respective vehicle is running.
In this respect, in such situations the bead offers particularly high resistance to detachment, with the result that the air fed into the operating chamber of the single-acting pneumatic cylinder-piston unit controlling the bead release tool is compressed to an increasing extent, and when said bead becomes detached said air behaves as a compressed spring undergoing instantaneous release.
Consequently the bead is forcibly urged towards the frame of the tyre removal machine, and undergoes bending and compression which can damage it, as has already happened.